diff --git a/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs b/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
index 934b780..aceb4a6 100644
--- a/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
+++ b/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
@@ -34,6 +34,8 @@
             message.SetExtension(OptionalBoolExtension, true);
             var serialized = message.ToByteArray();
 
+            MessageParsingHelpers.AssertWritingMessage(message);
+
             MessageParsingHelpers.AssertReadingMessage(
                 TestAllExtensions.Parser.WithExtensionRegistry(new ExtensionRegistry() { OptionalBoolExtension }),
                 serialized,